1. Field of the Invention
The present invention relates to a receiving circuit, and more particularly to a receiving circuit which permits simplification of its circuit arrangement and which further allows less power for a receiving system concurrent with reduction of power consumption.
2. Description of the Prior Art
One of important points needed for a receiving circuit in communications is to possibly reduce high-frequency circuit portions to thereby lessen a high power consumption factor, unstable operation factor and high manufacturing cost inherent in the high-frequency circuit, coupled with a space occupied by that circuit. These points are particularly important for mobile communication systems and others. For the reduction of the high-frequency circuit portions, a direct demodulation method has been proposed heretofore in terms of multiple frequency conversion and carrier frequency, thus accomplishing the direct conversion into a low frequency and the direct demodulation into a baseband. The aforesaid high-frequency circuit portion mainly constitutes a space diversity reception function necessitating two antenna systems.
As the direct demodulation method, there have been developed a number of methods in which a local oscillator generates a signal with a frequency equal to the carrier frequency which in turn, is mixed with a received input wave to derive a baseband signal therefrom. This direct demodulation method is made to produce a high-frequency signal with a frequency equal to the received signal frequency, and the high-frequency signal can be easy to release or emit in the air through an antenna of the receiver. Accordingly, another receiver adjacent thereto undergoes interferences to be inhibited from establishing communications. For this reason, this method has chiefly been adopted for communications based on frequency modulation methods which are relatively strong to single frequency interferences.
On the other hand, radio portable telephones, being recently rapidly put in widespread use, rely on a so-called PSK which is one amplitude transport modulation method, and the single frequency interference produces an offset in a demodulated output to deteriorate the error rate of a received signal. That is, since the local oscillation frequency can not take the carrier frequency, in this kind of communication method difficulty is encountered in direct frequency conversion and direct demodulation. One approach to resolve such a technical problem involves a method which, if a carrier frequency for a radio portable telephone is taken as fc and an offset frequency is taken to be fo, obtains fc+fo and fcxe2x88x92fo in order to provide a frequency-offset complementary local oscillation frequency for a frequency conversion. For carrying out this method, fc+fo and fcxe2x88x92fo are obtainable by multiplication process of fc and fo through a mixer (frequency mixer), while both signals fc+fo and fcxe2x88x92fo coexist in an output. More specifically, although the aforesaid process independently requires signals with the respective frequencies, the prior system can not practically satisfy this requirement. The prior system essentially employs filters for the respective frequencies, while suffering a disadvantage that a carrier frequency for a desired signal is variable and hence the filters need to designed to cope with the frequency variations.
The present invention has been developed with a view to resolving the prior problems, and it is therefore a general object of the present invention to provide a receiving circuit in a communication system with a plurality of channels, which is capable of lessening the power necessary in a receiving system and of reducing the power consumption with a simplified circuit.
Another general object of this invention is to provide a receiving circuit which can provide the frequencies fc+fo and fcxe2x88x92fo for elimination of the problems related to the aforementioned ordinary methods.
More specifically, an object of this invention is to provide a receiving circuit which performs a direction frequency conversion of a frequency in between the channels of a receiving system as a local frequency of a receiver and prevents the occurrence of the frequency offset in its output signal and the mixing of a signal of the adjacent channels thereinto.
In addition, a further object of this invention is to structurally review functional parts making up a receiving circuit to reduce the functional parts suffering from a large power consumption or replace them with different ones.
For these purposes, as one example, a receiving circuit according to this invention is composed of first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit for generating middle (intermediate) frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to said first frequently converting circuit and for outputting the downside frequency thereof as a conversion frequency input to said second frequency converting circuit, a common wave extracting circuit for extracting a component present in common in both outputs of the first and second frequency converting circuits, a frequency offset circuit for removing a frequency offset remaining in an output of the common wave extracting circuit, and a filter for removing an unnecessary frequency component remaining in an output of the frequency offset circuit.
In addition, in this invention, a means to resolve the prior problems is added to a local frequency complementary offset type direct frequency converting system, which bases this invention. A space diversity function can be realized with a receiving circuit based on a single direct quadrature detector.
With this arrangement, a received signal obtained from an antenna is fed to the first and second frequency converting circuits, while the local frequency signal generating circuit supplies, to the first and second frequency converting circuits, two different frequencies, i.e., the upside and downside frequencies corresponding to central values between channels, thus producing two output signals in terms of each of a desired wave, an upside channel and a downside channel. Further, a desired wave channel signal being a signal component present in common in both the first and second frequency converting circuits is extracted in the common wave extracting circuit. Since a frequency offset of xcfx89o remains in the output of the common wave extracting circuit, a minute frequency conversion is carried out in the offset frequency circuit to remove the offset amount. Moreover, an unnecessary frequency component generated in this process is removed through a filter, before the resultant signal is supplied as a baseband signal to a baseband signal processing section.
In accordance with a preferred form of this invention, a receiving circuit comprises first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second frequency converting circuits for generating middle frequencies between a ratio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first frequency converting circuit and for outputting the downside frequency thereof as a conversion frequency input to the second frequency converting circuit, a common wave extracting circuit for extracting a component present in common in both outputs of the first and second frequency converting circuits, a frequency offset circuit for removing a frequency offset remaining in an output o the common wave extracting circuit, and a filter for removing an unnecessary frequency component remaining in an output of the frequency offset circuit.
In another preferred form of this invention, a receiving circuit comprises first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second frequency converting circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first frequency converting circuit and for outputting the downside frequency thereof as a conversion frequency input to the second frequency converting circuit, a first frequency offset circuit for removing a frequency offset contained in an output of the first frequency converting circuit, a second frequency offset circuit for removing a frequency offset contained in an output of the second frequency converting circuit, a common wave extracting circuit for extracting a component present in common in both outputs of the first and second frequency offset circuits, and a filter for removing an unnecessary frequency component left in an output of the common wave extracting circuit.
In a further preferred form of this invention, a receiving circuit comprises first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second frequency converting circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first frequency converting circuit and for outputting the downside frequency thereof as a conversion frequency input to the second frequency converting circuit, first quantizing means for quantizing an output of the first frequency converting circuit, second quantizing means for quantizing an output of the second frequency converting circuit, a common wave extracting circuit for extracting a component present in common in both outputs of the first and second quantizing means, a frequency offset circuit for removing a frequency offset remaining in an output of the common wave extracting circuit; and a filter for removing an unnecessary frequency component left in an output of the frequency offset circuit.
In a further preferred form of this invention, a receiving circuit comprises first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second frequency converting circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first frequency converting circuit and for outputting the downside frequency thereof as a conversion frequency input to the second frequency converting circuit, first quantizing means for quantizing an output of the first frequency converting circuit, second quantizing means for quantizing an output of the second frequency converting circuit, a first frequency offset circuit for removing a frequency offset contained in an output of the first quantizing means, a second frequency offset circuit for removing a frequency offset contained in an output of the second quantizing means, a common wave extracting circuit for extracting a component present in common in both outputs of the first and second frequency offset circuits, and a filter for removing an unnecessary frequency components left in an output of the common wave extracting circuit.
In a further preferred form of this invention, a receiving circuit comprises first and second quadrature (orthogonal) demodulating circuits for receiving a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second quadrature demodulating circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first quadrature demodulating circuit and for outputting the downside frequency as a conversion frequency input to the second quadrature demodulating circuit, a first common wave extracting circuit coupled to the first and second quadrature demodulating circuits for extracting a component present in common in both I outputs of the fist and second quadrature demodulating circuits, a second common wave extracting circuit coupled to the first and second quadrature demodulating circuits for extracting a component present in common in both a Q output of the first quadrature demodulating circuit and a Q output of the second quadrature demodulating circuit reversed in polarity, a first frequency offset circuit for removing a frequency offset remaining in the I output extracted through the first common wave extracting circuit, a second frequency offset circuit for removing a frequency offset remaining in the Q output extracted through the second common wave extracting circuit, a first filter for removing an unnecessary frequency component remaining in an output of the first frequency offset circuit, and a second filter for removing an unnecessary frequency component remaining in an output of the second frequency offset circuit.
In a further preferred form of this invention, a receiving circuit comprises first and second quadrature demodulating circuits for receiving a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second quadrature demodulating circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first quadrature demodulating circuit and for outputting the downside frequency as a conversion frequency input to the second quadrature demodulating circuit, first and second frequency offset circuits for removing a frequency offset contained in common in I outputs and Q outputs of the fist and second quadrature demodulating circuits, a first common wave extracting circuit for extracting a component present in common in both the I outputs of the first and second frequency offset circuits, a second common wave extracting circuit for extracting a component present in common in both the Q output o the first frequency offset circuit and the Q output of the second frequency offset circuit 6I reversed in polarity, and first and second filters for removing unnecessary frequency components remaining in outputs of the first and second common wave extracting circuits.
In a further preferred form of this invention, a receiving circuit comprises first and second quadrature demodulating circuits for receiving a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second quadrature demodulating circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first quadrature demodulating circuit and for outputting the downside frequency as a conversion frequency input to the second quadrature demodulating circuit, first and second quantizing means for quantizing an I output and a Q output of the first quadrature demodulating circuit, respectively, third and fourth quantizing means for quantizing an I output and a Q output of the second quadrature demodulating circuit, respectively, a first common wave extracting circuit for extracting a component present in common in the I outputs of the first and third quantizing means, a second common wave extracting circuit for extracting a component present in common in the Q output of the second quantizing means and the Q output of the fourth quantizing means reversed in polarity, a first frequency offset circuit for removing a frequency offset remaining in the I output extracted through the first common wave extracting circuit, a second frequency offset circuit for removing a frequency offset remaining in the Q output extracted through the second common wave extracting circuit, a first filter for removing an unnecessary frequency component left in an output of the first frequency offset circuit, and a second filter for removing an unnecessary frequency component left in an output of the second frequency offset circuit.
In a further preferred form of this invention, a receiving circuit comprises first and second quadrature demodulating circuits for receiving a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second quadrature demodulating circuits for generating middle frequency between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first quadrature demodulating a circuit and for outputting the downside frequency as a conversion frequency input to the second quadrature demodulating circuit, first and second quantizing means for quantizing an I output and a Q output of the first quadrature demodulating circuit, respectively, third and fourth quantizing means for quantizing an I output and a Q output of the second quadrature demodulating circuit, respectively, a frequency offset circuit for removing frequency offsets remaining in the I outputs and the Q outputs of the first to fourth quantizing means, a first common wave extracting circuit for extracting a component present in common in the I outputs of the frequency offset circuit, a second common wave extracting circuit for extracting a component present in common in the Q output and a polarity-reversed Q output of the frequency offset circuit, a first filter for removing an unnecessary frequency component left in the I output extracted through the first common wave extracting circuit, and a second filter for removing an unnecessary frequency component left in the Q output of the second common wave extracting circuit.
Furthermore, according to this invention, in a receiving circuit, a correlator for calculating a mutual correction is used in placed of the common wave extracting circuit.
In a receiving circuit according to this invention, only the first frequency converting circuit is used for accepting the received signal, and after a frequency conversion by the first frequency converting circuit, a frequency-converted output of a side where no frequency conversion is effected by the first frequency converting circuit is obtained by another frequency converting circuit to attain two frequency-converted outputs necessary for the common wave extraction.
According to this invention, in a receiving circuit, only the first frequency converting circuit is used for accepting the received signal and only one of the first and second quantizing means is used for quantization, and after the quantization by the quantizing means, a frequency-converted output of a side where no frequency conversion is performed by the first frequency converting circuit is obtained by the digital frequency converting circuit to obtain two frequency-converted digital outputs necessary for the common wave extraction.
According to this invention, in a receiving circuit, only one of the first and second quadrature demodulating circuits is used for accepting the received signal, and two outputs of the quadrature demodulating circuit are frequency-converted by the frequency converted circuit to obtain a frequency-converted output of a side where no quadrature demodulation is made, to obtain two quadrature-demodulating outputs necessary for the common wave extraction.
According to this invention, in a receiving circuit, only one of the first and second quadrature demodulating circuits is used for accepting the received signal and only the second and third quantizing means are used, and after the quantization by the quantizing means, two outputs of the quantizing means are frequency-converted by the digital frequency converting circuit to obtain a frequency-converted output of a side no quadrature demodulation is effected, to obtain two quadrature-modulated outputs necessary for the common wave extraction.
Furthermore, in a preferred form of this invention, a receiving circuit comprises first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the first and second frequency converting circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first frequency converting circuit 2 and for outputting the downside frequency thereof as a conversion frequency input to the second frequency converting circuit, first and second integrating circuits, serving additionally as low-pass filters, for receiving outputs of the first and second frequency converting circuits through input lines, first and second buffer amplifiers for receiving outputs of the first and second integrating circuits, first and second transformers having the same construction and having primary coils whose one ends accepts outputs of the first and second buffer amplifiers, the other ends of the primary coils of the first and second transformers being grounded in the form of alternating currents, and secondary coils of the first and second transformers being coupled in parallel to each other in accordance with polarities of the primary coils and one ends of the secondary coils equal in polarity to the primary coils is used as an output terminal while the other ends of the secondary coils are grounded, a third buffer amplifier for accepting an output from the first and second transformers, a frequency offset circuit for removing a frequency offset remaining in an output of the third buffer amplifier, and a filter for removing an unnecessary frequency component remaining in an output of the frequency offset.
In a further preferred form of this invention, a receiving circuit comprises first and second frequency converting circuits for accepting a received signal obtained through an antenna, a local frequency signal generating circuit coupled to the fist and second frequency converting circuits for generating middle frequencies between a radio carrier frequency of the received signal and radio carrier frequencies of adjacent upside and downside channels and further for outputting the upside frequency of the upside and downside two wave frequencies as a conversion frequency input to the first frequency converting circuit and for outputting the downside frequency thereof as a conversion frequency input to the second frequency converting circuit, first and second differential amplifiers for accepting outputs of the first and second frequency converting circuits through input lines, first and second integrating circuits, serving additionally as low-pass filters, for correspondingly accepting outputs of the first and second differential amplifiers, first and second buffer amplifiers for supplying outputs of the first and second integrating circuits, means for making a feedback from an output side of each of the first and second buffer amplifiers to a negative input side thereof, first and second transformers having the same construction and having primary coils whose one ends accepts outputs of the first and second buffer amplifiers, the other ends of the primary coils of the first and second transformers being at least grounded in the form of alternating currents, and secondary coils of the first and second transformers being coupled in parallel to each other in accordance with polarities of the primary coils and one ends of the secondary coils equal in polarity to the primary coils is used as an output terminal while the other ends of the secondary coils are at least grounded in the form of alternating currents, a third buffer amplifier for accepting outputs of the first and second transformers, means for comparing an output of the third buffer amplifier with an average of the output of the first frequency converting circuit and the output of the second frequency converting circuit to correct the outputs of the first and second frequency converting circuits, means for causing an output of the third buffer amplifier to be feedbacked to one of the first and second frequency converting circuits to correct a difference between the outputs of the first and second frequency converting circuits, a frequency offset circuit for removing a frequency offset remaining in the output of the third buffer amplifier, and a filter for removing an unnecessary frequency component remaining in an output of the frequency offset circuit.
In accordance with this invention, in receiving circuit, the one end of the secondary coil different in polarity from the primary coil is connected with the third buffer amplifier in place of the end end of the secondary coil equal in polarity to the primary coil being connected therewith.
In accordance with this invention, in a receiving circuit, the one end of the secondary coil different in polarity from the primary coil is connected with the third buffer amplifier in place of the end end of the secondary coil equal in polarity to the primary coil being connected therewith.
A receiving circuit according to this invention further comprises local frequency generating means including a first frequency signal source for generating a signal with a frequency equal to a carrier frequency of a desired receiving signal, first means for accepting a signal from the first frequency signal source to phase-shift the accepted signal by a phase amount of xcfx80/2 with respect to a frequency of the accepted signal, a second frequency signal source for generating a frequency signal equal to xc2xd of a frequency channel spacing, second means for accepting a signal from the second frequency signal source to phase-shift a frequency of the accepted signal by a phase amount of xcfx80/2, a first quadrature modulator comprising a multiplier for receiving the two signals from the first and second frequency signal sources and a multiplier for receiving two signals from the first and second phase-shifting means, and a second quadrature modulator comprising two multipliers for receiving one of the two signals from the first and second frequency signal sources through the phase-shifting means.
A receiving circuit according to this invention further comprises local frequency generating means including a first frequency signal source for generating a frequency signal equal to a carrier frequency of a desired receiving signal, first means for accepting a signal from the first frequency signal source to phase-shift the accepted signal by a phase amount of xcfx80/2 with respect to a frequency of the accepted signal, a second frequency signal source for generating a frequency signal equal to xc2xd of a frequency channel spacing, second means for accepting a signal from the second frequency signal source to phase-shift a frequency of the accepted signal by a phase amount of xcfx80/2 multipliers for receiving the two signals from the first and second frequency signal sources, and means for inverting the polarity of an output of one of the multipliers to add the polarity-inverted output to an output of the other multiplier.
A receiving circuit according to this invention further comprises local frequency generating means including a frequency signal source for generating a frequency signal equal to a carrier frequency of a desired receiving signal, means for accepting a signal from the frequency signal source to phase-shift the accepted signal by a phase amount of xcfx80/2 in relation to a frequency of the accepted signal, a quadrature modulator comprising two multipliers for accepting a frequency signal from the frequency signal source through the phase shifting means, and means for inverting the polarity of an output of one of the two multipliers and for adding the polarity-inverted output to an output of the other multiplier.
In accordance with this invention, a receiving circuit includes means for analog-to-digital-converting first and second received signals obtained through one of the frequency converting circuits and the quadrature demodulating circuits, first and second Fourier transformers for accepting digital outputs of the analog-to-digital converting means, respectively, a correlator for accepting outputs of the first and second Fourier transformers at every frequency component, a weighting function device for accepting an output of the correlator, a weighting value multiplier for accepting an output of the weighting function device, an adder for accepting the outputs of the first and second Fourier transformers, and means for inputting an addition result of the adder to the multiplier, and an inverse Fourier transformer for accepting an output of the weighting value multiplier, wherein an inverse Fourier transformed output of the inverse Fourier transformer is used as a desired wave extraction result.
Moreover, in a preferred form of this invention, a receiving circuit comprises received signal inputting means for receiving a received signal from an antenna, a quadrature demodulator for carrying out a frequency converting process of the received signal from the received signal inputting means, first and second analog-to-digital converters for accepting an output signal of the quadrature demodulator to convert its analog signal into a digital signal, a sampling clock generator for generating, to the first and second analog-to-digital converters, a clock with a frequency which is more than two times a frequency corresponding to a bandwidth of the received signal, an adding circuit for adding a delayed pulse train (sequence) to a pulse train from the sampling clock generator, means for providing the pulse train from the sampling clock generator and the delayed pulse train as an sampling pulse for the analog-to-digital converters, and means for extracting a quadrature component of a desired receiving channel signal from digital output data of the analog-to-digital converters.
In a further preferred form of this invention, a receiving circuit comprises received signal inputting means for receiving a received signal from an antenna, a quadrature demodulator for carrying out a frequency converting process of the received signal from the received signal inputting means, first and second analog-to-digital converters for accepting an output signal of the quadrature demodulator to convert its analog signal into a digital signal, a sampling clock generator for generating, to the first and second analog-to-digital converters, a clock with a frequency higher than a frequency corresponding to a bandwidth of the received signal, an adding circuit for adding a delayed pulse train to a pulse train from the sampling clock generator, means for providing the pulse train from the sampling clock generator and the delayed pulse train as an sampling pulse for the analog-to-digital converters, and means for extracting a quadrature component of a desired receiving channel signal from digital output data of the analog-to-digital converters, wherein one or more delayed pulse trains is generated so that the pulse train from the sampling clock generator and the delayed pulse train from the delayed pulse train adding circuit take a delayed time other than a phase difference corresponding to xcfx80 of a frequency of the desired channel signal.
In accordance with this invention, in a receiving circuit, the pulse train is generated from the sampling clock generator and the delayed pulse train taking a phase difference time corresponding to xcfx80/2 relative to a frequency of the desired channel signal is generated from the delayed pulse train adding circuit.
In a further preferred form of this invention, a receiving circuit comprises received signal inputting means for receiving a received signal from an antenna, first and second analog-to-digital converters for accepting the received signal to convert the received signal into a digital signal, a sampling clock generator for generating, to the first and second analog-to-digital converters, a clock with a frequency higher than a frequency corresponding to a bandwidth of the received signal, an adding circuit for adding a delayed pulse train to a pulse train from the sampling clock generator, means for providing the pulse train from the sampling clock generator and the delayed pulse train as an sampling pulse for the analog-to-digital converters, and means for extracting a desired receiving channel signal from digital output data of the analog-to-digital converters.
Furthermore, in accordance with this invention, in a receiving circuit, in addition to the pulse train from the sampling clock generator, the delayed pulse train taking a phase difference time corresponding to xcfx80/2 with respect to a frequency of the desired channel signal is generated from the delayed pulse train adding circuit.
Moreover, in accordance with this invention, in a receiving circuit, in addition to the pulse train from the sampling clock generator, a polarity of delayed pulse trains taking a phase difference time corresponding to xcfx80/2 in relation to a frequency of the desired channel signal are generated from the delayed pulse train adding circuit.
In a further preferred form of this invention, a receiving circuit comprises received signal inputting means for receiving a received signal from an antenna, an analog-to-digital converter for accepting the received signal to convert the received signal into a digital signal, a sampling clock generator for generating, to the analog-to-digital converter, a clock with a frequency higher than a frequency corresponding to a bandwidth of the received signal, an adding circuit for adding a delayed pulse train to a pulse train from the sampling clock generator, means for providing the pulse train from the sampling clock generator and the delayed pulse train as an sampling pulse for the analog-to-digital converter, and means for extracting a desired receiving channel signal from digital output data of the analog-to-digital converter.
In a still preferred form of this invention, a receiving circuit comprises received signal inputting means for accepting received signals from a plurality of antennas, first and second frequency converting means for accepting the received signals, a local oscillator for supplying, to the first and second frequency converting means, an output at a frequency produced by providing a frequency offset of xc2xd of a channel spacing frequency to a desired carrier frequency, first and second analog-to-digital converters for accepting signals from the first and second frequency converting means, a sampling clock generator for generating, to the analog-to-digital converters, a clock with a frequency higher than a frequency corresponding to a bandwidth of the received signal, an adding circuit for adding a delayed pulse train to a pulse train from the sampling clock generator, means for providing the pulse train from the sampling clock generator and the delayed pulse train as an sampling pulse for the analog-to-digital converters, and means for extracting a desired receiving channel signal from digital output data of the analog-to-digital converters.
In accordance with this invention, in a receiving circuit, the local oscillator for providing the output to the first and second frequency converting means is provided independently so that local oscillation frequencies are frequencies produced by providing positive and negative frequency offsets of xc2xd of a channel spacing frequency with respect to the desired carrier frequency.
Furthermore, in accordance with this invention, in a receiving circuit, the two received signals are supplied to the first and second analog-to-digital converters without being frequency-converted.